The mainstream in conventional brake control methods for electric railcars is the “electropneumatic-blending brake control method” that uses an electric brake and a pneumatic brake in combination. Under this method, when speed decreases below the required value, regenerative (electric) braking force will decrease in steps. Conversely, pneumatic braking force will gradually increase at the same time until the electric railcar has stopped, and the electric railcar will finally stop only by means of the pneumatic brake. Change-over from the electric brake to the pneumatic brake is controlled so that the total braking force of both brakes becomes constant, and thereby, approximately constant retardation force is maintained until the electric railcar has stopped.
The brake control method usually adopted for recent electric railcars, however, is the “fully-electric brake final-velocity control (electric stopping brake control)” method in which regenerative (electric) braking force is exerted until the electric rail car has stopped. This method is very valid for realizing accurate stopping position control during automatic operation, since highly accurate torque response characteristics can be maintained even in the vicinity of speed zero.
This fully-electric brake final-velocity control (electric stopping brake control)” method is actually realized for the control apparatus for an electric railcar, described in, for example, Japanese Application Patent Laid-open Publication No. 2001-251701.
Under the electropneumatic-blending brake control method, regenerative (electric) braking force starts decreasing in steps from a speed of about 10 km/h, and at a speed of at least 1 km/h, the regenerative (electric) braking force completely decreases to zero. That is to say, regenerative (electric) braking force is not completely exerted in the vicinity of speed zero, such as speeds less than 1 km/h.
Under the fully-electric brake final-velocity control (electric stopping brake control) method, however, the desired regenerative (electric) braking force is exerted until the vicinity of speed zero has been reached, and the selection of the optimal regenerative (electric) braking force is abruptly started in the region from the vicinity of speed zero towards speed zero. That is to say, regenerative (electric) braking force is exerted in the vicinity of speed zero, such as speeds less than 1 km/h.
Most of recent electric railcars employ motor torque control based on PWM inverter control, and when the presence/absence of braking force in the vicinity of speed zero during regenerative (electric) braking is considered, the following problems can be assumed:
When the regenerative (electric) brake continues to operate until the electric railcar has stopped, brake mode control is transferred to a negative-phase brake mode (reversing/power driving mode) through an inverter frequency zero point. During passage through this inverter frequency zero point, there may occur the current concentration in which switching elements of either a U-, V-, or W-phase maintain a high conduction ratio. Heat loss due to the current concentration can cause abrupt increases in the temperatures of the switching elements, hereby accelerating the deterioration thereof or according to conditions, leading to element destruction.